feat: throw when inputContainer refers to an <input>

[sarahdayan]

We generate the search box for users, but it's likely that they will try to integrate Unified in a website that already has one and will try to hook inputContainer to it. We mention it in the documentation, but we can't rely on this alone, as this is a likely error and it will make Unified fail from the very first try, causing a pretty bad experience.

We've seen that error during user testing, and it results in Preact throwing way too deep for the error to be understandable. This PR introduces a custom error that makes it clear what is happening and what steps to take from there.

[francoischalifour]

Ideally this kind of checks should be in a runtime validator step but we don't have any yet.

[sarahdayan]

Ideally this kind of checks should be in a runtime validator step but we don't have any yet.

I agree. We can definitely build a runtime validator that runs during development and throws descriptive errors, but disappears in production. I can take care of this in another PR. wdyt?

[francoischalifour]

Yeah that'd be great!